The invention concerns a hydraulic steering device for vehicles, in particular construction machines.
Hydraulic steering devices are known in a multitude of forms and are widespread. Usually, a linear cylinder is employed in combination with steering rods in order to bring the wheels to be controlled into the desired steering angle. For controlling the linear cylinder, arrangements of control valves and other components are usual, which assure a precisely defined flow of pressure medium into the cylinder.
A particular system of pumps for a hydraulic steering device is disclosed in DE 101 04 851 A1. This system includes of a vane pump that possesses a controllable pumping capacity and hence can provide a variable flow of pressure medium for the steering device. On this pump the pumping capacity is varied by means of a movable curved ring, the position of which is determined by an electric motor. A controller measures the current position of the curved ring and from this generates the necessary positioning signals for the electric motor. Because of its construction, this vane pump can, of course, produce only moderate maximum pressures and is therefore not suitable for all types of hydraulic steering actuators.
Additional known hydraulic steering systems possess a series of disadvantages. The use of linear cylinders is unsuitable, particularly for articulated vehicles, since they can be damaged in rough working environments. Furthermore, the necessary mechanism is expensive in construction.
The valve arrangements necessary for the steering device are likewise very expensive. Various hydraulic components generally combined in a servostat comprise different hydraulic elements, usually, a rotor unit which is mechanically connected with the steering valve, a pressure relief valve for high-pressure protection, non-return valves and some further valves in order to assure a supply of pressure medium to the steering unit under all operating conditions. The steering device must function always, even in emergency situations and with the main engine switched off, so that usually in addition an emergency pressure supply is available, e.g. an electric motor or something similar is provided. The throttling edges in the valves have inherent pressure losses, which reduce the efficiency of such an arrangement. Electro-hydraulic controls for the steering device, in which mechanical actuation of the rotor unit in the servostat can be dispensed with, are very expensive with these solutions, as complicated electro-hydraulic valve arrangements must be employed.
It would be a significant advantage to provide a hydraulic steering device for vehicles that overcomes the various problems and difficulties described above.